A piston is a car parts
of reciprocating engines, pumps and gas compressors. It is located in a
cylinder and is made gas-tight by piston rings. In an engine, its
purpose is to transfer force from expanding gas in the cylinder to the
crankshaft via a piston rod and/or connecting rod.

In a pump, the
function is reversed and force is transferred from the crankshaft to the
piston for the purpose of compressing or ejecting the fluid in the
cylinder. In some engines, the piston also acts as a valve by covering
and uncovering ports in the cylinder wall.

Piston engines

1)Internal combustion engines

There are two ways that an internal combustion piston engine can
transform combustion into motive power. These are the 2-stroke cycle
and the 4v-stroke cycle. A single cylinder two-stroke engine produces
power every crankshaft revolution, while a single cylinder four-stroke
engine produces power once every two revolutions. Older designs of small
two-stroke engines produced more pollution than four stroke engines.
However, modern two-stroke designs, like the Vespa ET2 Injection utilise
fuel-injection and are as clean as four-strokes.

Large diesel two-stroke
engines, as used in ships and locomotives, have always used fuel
injection and produce low emissions. It is bigger than most two-story
houses, has pistons nearly 1 metre in diameter and is one of the most
efficient mobile engines in existence. In theory, a four-stroke engine
has to be larger than a two-stroke engine to produce an equivalent
amount of power. Two-stroke engines are becoming less common in
developed countries these days, mainly due to manufacturer reluctance to
invest in reducing two-stroke emissions. Traditionally, two-stroke
engines were reputed to need more maintenance.

Even though the simplest
two-stroke engines have fewer moving parts, they could wear out faster
than four-stroke engines. However fuel-injected two-strokes achieve
better engine lubrication, and cooling and reliability should improve
considerably.